During delivery of hot tap water in district heating station, a primary flow of centrally heated water, which is conducted into a heat exchanger, where a secondary flow of hot tap water is heated to a constant consuming temperature in the heat exchanger. Control of the constant consuming temperature on the secondary side have been obtained in the district heating station, either through automatic mechanical, or through electronic control devices, which control the temperature on the basis of correction of the difference between desired and actual outbound temperature on the secondary side through feedback temperature measurement from the secondary side. Whenever electronic control devices are used, PI or PID regulators are commonly used, which control the flow on the primary side by, depending on the present outbound temperature on the secondary side, closing or opening a valve on the primary side. Thus, the heating effect on the primary side is regulated, so that the desired outbound temperature on the hot tap water is obtained.
Both the mechanical and the electronic systems exhibit drawbacks, since the control is not as fast as would be desired, whereby there may be a delay before the correct outbound temperature is reached on the secondary side. This entails a lag before the correct temperature is obtained at the tap location of the secondary circuit, and, in the worst case, a risk of scalding.
Another drawback is that an oscillation in the control easily arises, since it is, in practise, impossibly to optimise the regulating equipment with respect to all occurring operating conditions. The conducting temperature and difference pressure of the district heating system, i.e., the primary side, varies during the year and along the path of the district heating line.
The pressure fluctuations in the district heating system are partly dependent on the present distance from the heat source, partly on the relative position of the district-heating central in the system. The statically programmed characteristics of the regulators cannot be optimised with respect to all occurring operation scenarios, which entails, among other things, oscillations of the outbound temperature during certain operating conditions. The temperature oscillations entail e.g. the following potential drawbacks.
Poor comfort at tap locations with a small smoothening effect from the line system, which I particularly noticeable in single household residential property.
Increased calcification of heat exchangers when temperatures above 60° C. are reached. Increased wear of regulating members.
Impaired cooling of the district heating system, which may entail large production costs.
A system is previously known from U.S. Pat. No. 5,363,905, where a feedback temperature from the secondary side is used to affect the regulatory valve on the primary side. This type of solution corrects different pressures on the primary side, but it does not provide the desired rapid correction of the temperature during fluctuations in the flow of hot tap water on the secondary side. In this case, measuring of the pressure drop over a constriction in the primary circuit is used, as well as pressure measurement on the primary side, but also measurement of the temperature before and after the heat exchanger on the primary side. This system becomes relatively expensive, since two pressure gauges are needed on the primary side, and can not readily provide rapid regulation of the temperature on the secondary side upon sudden changes of the tap flow on the secondary side. The regulatory measures will not come into action until the temperature actually drops on the secondary side, and the typical oscillations of the hot tap water temperature are obtained.
EP 0,526,884 discloses a regulatory technique from thermo printers, in which the write head is controlled to a constant temperature, primarily by regulating the electric energy delivered to the print head, and secondarily by an adjustably compensating coolant flow. The temperature of the thermal head is measured by a first temperature sensor, and the temperature of the removed coolant fluid is measured by a second temperature sensor. The system calculates the removed heating capacity in the coolant flow by measuring and regulating the coolant flow, and measures the temperature of the inbound, as well as the outbound (heated), coolant flow.
Through WO 96/17210, a control system for a district heating plant is previously known, in which is comprised temperature measurements, and measurement of the flow on the primary side, with the purpose to provide the desired control, and to calculate the consumed power to be billed to the customer. Also in this case, flow measurement was not used on the secondary side, meaning that the system likely is subjected to oscillations in the temperature of the outbound water on the secondary side.
DE U 1 296,17,756 discloses a system, in which a shunting control is carried out on the primary side, with a feedback, on the primary side, of outbound flow from the heat exchanger, back to the inbound flow of the heat exchanger. Here, the assumption is made that, if the temperature of the outbound flow on the primary side from the heat exchanger is kept constant, a constant temperature of the hot tap water on the secondary side will be obtained. This assumption unconditionally entails that oscillations of the temperature on the secondary side are obtained, since the surfaces of the heat exchanger first must be cooled by the hot tap water. Further, the system does not respond rapidly to abrupt increases in the flow of hot tap water, since the regulation is not effected until the temperature on the primary side has dropped.
The prior art has not recognised the need to take rapid action when the heat consumption on the secondary side abruptly changes, i.e., when the flow is altered incrementally. This means that the control systems often end up in oscillating conditions, as far as the temperature on the secondary side is concerned. In spite of the multitude of separate solutions to the partial problems, no system has exhibited characteristics, simultaneously allowing a stabile function regardless to the location in the district heating system. Most systems in which the temperature is to be carefully controlled on the secondary side have comprised regulatory loops with feedback information concerning the current temperature value, whereby countermeasures are taken against the deviation of the measured value of the outbound temperature from the set point value of the same. Thus, this system is based on action in dependence on the error in the resulting outbound temperature. Such a system must wait until an error can be detected before countermeasures can be taken, which entails a delay of the time point when the actual desired heat consumption is altered.